1.select * from users order by cdate limit 10;

2.select b.id,b.title,b.pcount,u.name from blog b,users u where b.uid=u.id order by b.pcount desc limit 5;

3.select u.id,u.name,count(*) from users u,blog b where b.uid=u.id group by b.uid order by count(*) desc;

4.select u.id,u.name,avg(b.pcount) from users u,blog b where b.uid=u.id group by u.id order by avg(b.pcount) desc limit 3;

5.delete from users where id not in(select uid from blog group by uid);